If buying and holding is making you nervous, then maybe you are risking too much. Don't wait until you get nervous though. Make adjustments to your balances at the end of each year or twice or year and stick to it.
I now put 24% into retirement. I invest the majority in cheap index funds, but I have never been 100% in stocks. I always have put a substantial portion in investments with fixed returns just for peace of mind. As I get closer to retirement, I now only put 50% in stocks. I avoid individual stocks, because I think you need to follow the market closer than I do in order to make smart choices. If you feel strongly about a stock, maybe invest a small percentage in it.